crucible: fs: use a much smaller default search buffer size

It turns out we never use a value for m_buf_size that isn't the default,
and we also never ask for more than a few thousand items; however,
we do spend a ton of time memsetting the huge buffer to zero.

I don't know what the ideal size is, but 16K is a far better guess
than 1MB.  Let's reduce it for some immediate CPU benefit, and determine
what the size should be later.

Reported at https://github.com/Zygo/bees/issues/11

include/crucible/fs.h

@@ -156,7 +156,7 @@ namespace crucible {
 	ostream & operator<<(ostream &os, const BtrfsIoctlSearchHeader &hdr);
 
 	struct BtrfsIoctlSearchKey : public btrfs_ioctl_search_key {
-		BtrfsIoctlSearchKey(size_t buf_size = 1024 * 1024);
+		BtrfsIoctlSearchKey(size_t buf_size = 16 * 1024);
 		virtual bool do_ioctl_nothrow(int fd);
 		virtual void do_ioctl(int fd);